The removal of NOx from the exhaust gases of internal combustion engines is required for cleaner operating vehicles. Improvements in fuel efficiency are achieved by operating at conditions with an excess of air than required for stoichiometric combustion (i.e., lean burn or rich conditions). Such “lean burn” conditions are commonly achieved in diesel engines and four cycle engines. However when lean-burn conditions are employed, common pollution reduction devices (e.g., three-way catalysts) are inefficient in the reduction of nitrogen oxides.
One approach to reduce nitrogen oxide pollutants in exhaust gases of engines operating under lean-burn conditions has been to incorporate a non-thermal plasma reactor in the exhaust lines along in addition to a catalyst converter formulated for the NOx reduction. Such reactors treat the exhaust gases using a non-thermal plasma field. The plasma converts NO to NO2, the NO2 must then be subsequently reduced by a selective catalyst. For example, a non-thermal plasma reactor is described in U.S. Pat. No. 6,139,694, the contents of which are incorporated by reference herein.
Non-thermal plasma reactors include a non-thermal plasma-generating substrate (“substrate”) disposed within a housing. The substrate includes a plurality of dielectric plates each being spaced from one another to form a plurality of exhaust gas flow channels. Preferably, the dielectric plates are non-conductive materials such as quartz, glass, alumina, mullite, and oxide free ceramics (e.g., silicon nitrite, boron nitrite, aluminum nitrite). A voltage supply is connected to a pair of electrodes on each dielectric plate for providing a voltage between the dielectric plates in order to generate the plasma field in the flow channel between the plates. The exhaust gas flows through the flow channel, exposing the gas to the plasma field. The plasma field converts NO into nitrogen dioxide or NO2.
The dielectric plates are prone to crushing from forces applied to the surface of the plates parallel to the flow passage due to the thin cross section of the plates and due to the fact that they are only supported at two sides of the passage. Thus, the forces necessary to restrain the substrate in the housing may damage the unsupported surface areas and may bend or deform the outer plates into the flow passage. The substrate must be isolated from the housing to prevent high voltage arcing from a buss line termination on the substrate to the housing. Moreover, the substrate is subject to heating and cooling cycles, which places an additional strain on the substrate. These factors and others create obstacles with respect to retaining the substrate in the reactor.